In the field of battery systems, there are many instances where it is preferable or necessary to keep track of the amount of power available in a battery before, during, and after the battery is subject to an external load. If the amount of battery power is effectively monitored, users are able to monitor further use of the battery and replace or recharge a depleted battery before a minimum useful power threshold is reached or the battery is completely discharge. As those of skill in the art realize, the ability to continuously monitor the amount of power remaining in a power source is almost a necessity in various end-use applications, such as military applications. Proper monitoring of battery power is also of concern when the battery remains unused for many years.
Some end-use applications, such as military applications, employ a battery with a built-in protection diode (designated as D3 in FIG. 2) and a fuse of conventional design. The battery's protection diode is typically a high-current Schottky diode. The protection diode and fuse are in electrical connection with the battery and serves to protect the battery from electrical surge events or related harmful events (attempts to recharge a primary battery). Most electronic devices on the market today, which consume battery power, incorporate such a protection diode in series with a fuse. If no protection diode or fuse exist on the battery, these can usually be added with minimal impact to the battery. Further, due to additional power requirements, some end-use applications require more than one battery. When one or more batteries in electrical connection with each other are used, the combination is generally known as a "battery string". Those skilled in the art know that the term "battery" includes one or more batteries.
Most users of a battery, however, do not take advantage of all power remaining in the battery because there is no convenient, economical or highly accurate way to monitor the power level in the battery. Rather, users are apt to replace the battery or continuously recharge them in an attempt to maintain adequate power for an end-use application (or "load"). Successive replacement of batteries prior to consumption of all available power is cost inefficient. Continuously recharging the batteries may result in what is known as the "memory effect", wherein the battery's ability to retain charge diminishes significantly over time.
Portable devices exist for measuring the charge capacity available on a battery. However, such measuring devices are expensive, are usually larger than the batteries themselves, and do not provide the level of accuracy required for many end-use applications (such as, for example, military equipment).
Smaller devices exist that are capable of monitoring the discharge rate of a battery. An example of such a battery capacity discharge monitor is disclosed in U.S. Pat. No. 5,372,898 to Atwater et al. A diagram of the Atwater et al. device is shown in FIG. 1. The device of Atwater et al. includes a sense resistor of minimal resistive force, sense circuitry, counter circuitry and miscellaneous logic circuitry. In order to determine the amount of power capacity remaining in a high-powered battery system, the Atwater et al. device monitors the current flow from the battery to an attached load through the sensing resistor. In operation, the sensing resistor is placed in serial electrical connection with the battery (or batteries connected in series). When voltage is detected across the sensing resistor due to the presence of a load on the battery, the voltage signal is then amplified and integrated over time to obtain an average current value. The resulting integrated signal is then applied to a capacitor in the miscellaneous logic for storage. Whenever a predetermined threshold voltage is achieved across the capacitor, the sense circuitry operates to discharge the capacitor and increase the value in the counter circuitry. When the capacitor is discharged, this event represents the discharge of one coulomb of power from the battery. Atwater et al.'s counter circuitry maintains count of the number of discharge cycles (or, the number of coulombs of power removed from the battery). The counter circuitry is electrically connected to light emitting diodes to indirectly represent the amount of power dissipated from the battery.
Circuitry that senses the discharge rate from a battery, such as that disclosed by Atwater et al., is deficient for several reasons. First, a power-discharge sensing circuit contributes greatly to the amount of power dissipated from the battery being monitored because the voltage across the sensing resistor must be continuously monitored. While this problem may not exist with high-power batteries, problems do exist in those instances where the end-use application requires lower power batteries.
Second, high-power discharge sensing circuits, like that disclosed in Atwater et al., require active (e.g., always "ON") circuitry to enable the sense circuitry and counter circuitry to operate. Such active circuitry is purposely designed to continuously draw power from the battery, and is not easily disabled because the circuitry is located in a housing with the battery. Thus, in order to operate, these circuits require continuous power from the same power source these circuits are designed to monitor. Such continuous consumption of power contributes or greatly reduces the operable life of the battery. Third, these types of battery sensing methods incorporate electronic components which draw a large amount of current from the battery, whether a load is placed on the battery or not. Finally, discrete MOSFETs used in these circuits are expensive.